Wrong "Too many indexers" when value in series with MultiIndex is None

[ludaavics]

Code Sample, a copy-pastable example if possible

import pandas as pd
s = pd.Series([None, 'ad', 'bc', 'bd'], index=pd.MultiIndex.from_product([['a', 'b'], ['c', 'd']]))
s.loc['a', 'c']  # raises IndexingError: Too many indexers
s.loc(axis=0)['a', 'c']  # workaround

Problem description

Similar to #14885 but, in our case, the key is indeed present in the Multi Index (but the associated value is None).
Credit for the workaround goes to @bluenote10

Expected Output

Nothing (i.e. None object)

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Darwin
OS-release: 17.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.4
pytest: 4.0.0
pip: 18.1
setuptools: 39.1.0
Cython: None
numpy: 1.14.3
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: 1.8.1
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: 2.6.4
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: 1.1.0
xlwt: None
xlsxwriter: 1.0.4
lxml: None
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[gfyoung]

I might consider adding this example to the existing issue, given how similar they are.

What do you think?

cc @jreback

[jreback]

When accessing a MI tuple you almost always need to specify all axes as a matter of course, see the documentation: http://pandas-docs.github.io/pandas-docs-travis/advanced.html#using-slicers

In [8]: s.loc[('a', 'c'), :]
Out[8]: 
a  c    None
   d      ad
dtype: object

If documentation is unclear, certainly would take PR to update.

[ludaavics]

@jreback I understand your comment on dataframes, but not on series: there isn't really any ambiguity about which column we want? But point well taken.
Also -- wouldn't we expect s.loc[('a', 'c'), :] (or whatever other appropriate syntax) to return just None?